Several private households are not connected to the public sewage system and associated purification plants, and they use various types of mini purification plants for processing the waste water. Such mini purification plants are often simple “septic tanks” using anaerobic fermentation at low temperatures which is a slow process and where the outlet often ends in a cesspool, viz. a particular type of receptacle from which the water gradually percolates into the surrounding layers of soil. In Denmark, more than 300,000 houses suitable for use throughout the year process the waste water solely by means of septic tanks. However, the plants can also be of the type “immersed aerated bio filter plant”, several different types thereof being available on the market. All of the latter types share the feature of having the entire biological process carried out in a single section which results in unpractical operation and sometimes in substantial variations in efficiency. Specifically, it is often impossible to meet the requirements presented to the nitrification of the purified waste water, viz. to an increased organic purification.
The above is due to the fact that the nitrifying autotrophic bacteria constantly compete with the heterotrophic bacteria, viz. bacteria using organic matter as a carbon source, and to the fact that the autotrophic bacteria therefore have difficulties in surviving because they grow substantially slower than the heterotrophic bacteria, viz. suffer from a long half-life, and by a much lower yield constant. The heterotrophic bacteria produce approximately 10 times as much sludge per energy unit as the nitrifying bacteria. This sludge takes up much space sand further increases the space competition additionally.
When observing the flow of water through a bio filter, it is obvious that the heterotrophic bacteria are the first bacteria to colonise at the beginning of the filter. Whether or not space is left for the nitrifying bacteria depends on the concentration of organic matter in the inlet of the bio filter.
The present invention is based on the assumption that it is possible to improve the conditions for the nitrifying bacteria by separating the step of decomposing organic matter from the nitrifying step in a method of biologically purifying waste water. A simultaneous preliminary aeration for stripping off H2S and other gases renders it possible to obtain an improved purification of the waste water in the biological steps of the process. Often, the gases formed in the anaerobic section of a septic tank have an inhibiting effect on the biological purification of the waste water. When carrying out the method according to the invention in pilot scale, it turned out that the pH-value increases across the plant when a preliminary aeration is carried out rather than decreases as expected due to the nitrification. Such a processing can only be explained by the fact that an acid is removed by the aeration process and the fact that said acid is H2S. Therefore an improved biological activity is obtained in the subsequent step where the water is now at least partly purified of H2S. Thus, by aerating the water and thereby stripping off H2S and other gases it is rendered possible to obtain an improved purification of the waste water in the biological section of the mini purification plant. In addition, it is rendered possible to remove the many bactericides, viz. chlorine, acetone etc., which can be present in waste water discharged from households whereby the bacterial flora of the system is subjected to a lenient processing and thereby kept efficient.